Non-cyclic saturated esters are known to be useful as fuel components. Ethyl pentanoate is known as a gasoline component, for example, and pentyl pentanoate (also known as pentyl valerate) is a diesel component. Non-cyclic saturated esters of this type may be formed by hydrogenation of a reactant such as a lactone or a carboxylic acid or ester having a gamma-carbonyl group. Such reactants are available from biomass, in particular from cellulose feedstock material, rendering their use in the preparation of ester fuel components commercially attractive.
The catalytic hydrogenation of lactones, esters and carboxylic acids which have gamma-carbonyl groups using a strongly acidic heterogenous catalyst comprising a hydrogenating metal on a zeolite base is described in WO-2006/067171. This process may be used to convert gamma valerolactone (4-pentalactone or GVL) into valeric (pentanoic) acid and its esters.
Gamma valerolactone (GVL), which may itself be prepared by catalytic hydrogenation of levulinic acid or its esters, as described in WO-2006/067171, U.S. Pat. No. 5,883,266, WO-02/074760, WO-98/26869 and EP-0 069 409, is known to be a very stable compound. As described in WO-2006/067171, GVL is more easily formed under catalytic hydrogenating conditions than non-cyclic hydrogenated compounds such as pentanoic (valeric) acid or pentanoates.
Further WO-2008/142127 describes a process for converting levulinic acid into pentanoic acid comprising two hydrogenating steps. In the second hydrogenation step an effluent comprising gamma valerolactone is contacted, under hydrogenating conditions and in the presence of hydrogen, with a strongly acidic catalyst and a hydrogenation metal. In experiment 3 WO-2008/142127 describes a process wherein a feedstock of pure gamma valerolactone is reacted over a catalyst containing platinum on an acidic carrier of 25 wt % surface dealuminated ZSM-5 and 75 wt % silica binder. As illustrated in table 3, the use of a catalyst comprising Pt on an acidic carrier of surface-dealuminated ZSM-5 and silica binder results in a product composition containing mainly unconverted gamma valerolactone and pentanoic acid and only small amounts of the pentyl pentanoate ester (0.4-3.2 mol %).
There therefore remains a continuing need for an improved process to prepare esters useful as fuel components in one direct step.